diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -3,7 +3,7 @@
 #
 # Warning flags for compiling the kernel and components of the kernel:
 #
-CWARNFLAGS?=	-Wall -Wnested-externs -Wstrict-prototypes \
+CWARNFLAGS?=	-Wall -Wstrict-prototypes \
 		-Wmissing-prototypes -Wpointer-arith -Wcast-qual \
 		-Wundef -Wno-pointer-sign ${FORMAT_EXTENSIONS} \
 		-Wmissing-include-dirs -fdiagnostics-show-option \
diff --git a/sys/conf/kern.pre.mk b/sys/conf/kern.pre.mk
--- a/sys/conf/kern.pre.mk
+++ b/sys/conf/kern.pre.mk
@@ -225,7 +225,6 @@
 	-Wno-duplicate-decl-specifier \
 	-Wno-missing-braces \
 	-Wno-missing-prototypes \
-	-Wno-nested-externs \
 	-Wno-parentheses \
 	-Wno-pointer-arith \
 	-Wno-strict-prototypes \
diff --git a/sys/conf/kmod.mk b/sys/conf/kmod.mk
--- a/sys/conf/kmod.mk
+++ b/sys/conf/kmod.mk
@@ -551,8 +551,7 @@
 	-I${SYSDIR}/cddl/compat/opensolaris \
 	-I${SYSDIR}/cddl/contrib/opensolaris/uts/common \
 	-include ${ZINCDIR}/os/freebsd/spl/sys/ccompile.h
-OPENZFS_CWARNFLAGS= \
-	-Wno-nested-externs
+OPENZFS_CWARNFLAGS=
 
 .include <bsd.dep.mk>
 .include <bsd.clang-analyze.mk>
diff --git a/sys/modules/pms/Makefile b/sys/modules/pms/Makefile
--- a/sys/modules/pms/Makefile
+++ b/sys/modules/pms/Makefile
@@ -22,7 +22,6 @@
 
 CFLAGS+=-Wredundant-decls
 CFLAGS+=-Wunused-variable
-CFLAGS+=-Wnested-externs
 CFLAGS+=-Woverflow
 CFLAGS+=-Wmissing-prototypes
 CFLAGS+=-Wparentheses